Download Metamorphic rocks lab I

Name _______________________
Petrology — Spring 2006
Metamorphic rocks lab — Part I
Barrovian metamorphism
Due Tuesday 4/13
Learn to identify the metamorphic minerals common in Barrovian metamorphism of pelitic rocks.
Take note of metamorphic textures and develop a sense for the sequence of metamorphic mineral
growth and deformation.
Chlorite zone (chloritoid subzone )
#73. Chloritoid phyllite, of Paleozoic age, Vermont. Phyllite with distinct, green porphyroblasts
of chloritoid.
Quartz — 35
Muscovite — 39
Chloritoid — 20: Euhedral porphyroblasts, pleochroic in blue and blue-green. Can be confused
with chlorite or amphibole, but it has much lower birefringence than amphiboles. It lacks the
micaceous cleavage of chlorite and can be distinguished by its polysynthetic twinning
Chlorite — ≥ 5: Compare its distinguishing features and habit with chloritoid
Ilmenite and pyrite — 1: The pyrite is locally altered to hematite
Sphene?: very small crystals with ilmenite
Stilpnomelane (??): is more probably a weathering product that has formed on muscovite.
Is the chloritoid pre-, syn-, or post-kinematic [i.e. - did it grow before, during, or after the
deformation which produced the schistosity of this rock]?
Biotite zone
#74. Biotite schist, of Paleozoic age, Belfast area, Maine.
Quartz — 60
Muscovite — 10
Biotite — 20: Porphyroblastic
Tourmaline — trace
Chlorite — 5-10
Apatite — trace
Garnet (spessartine): a trace in slides a, b, c, d, and f. Don't necessarily expect to find it without
an exhaustive search! Why does this rock not belong to the garnet zone, given the presence of
garnet in these slides?
Name _______________________
Petrology — Spring 2006
Garnet zone (More properly, the almandine zone)
#75. Garnet phyllite, of Paleozoic age, Belfast area, Maine. Metamorphic grade transitional
with amphibolite-facies.
Quartz — 50
Muscovite — 8
Biotite — 20
Garnet — 20: almandine
Chlorite — 2
This rock superficially resembles #65, but notice that this rock contains muscovite, whereas #65
contains calcite and abundant chlorite. This rock is pelitic and #65 is not.
Staurolite zone
#83. Staurolite schist, of Paleozoic age, Belfast, Maine.
Quartz — 40
Muscovite — 20
Biotite — 10: altered to chlorite
Chlorite — 10: replaces biotite and staurolite. What is the approximate ratio Fe/(Fe+Mg) of the
Chlorite?
Staurolite — 20: weak pleochroism: colorless and pale yellow. Some has anomalous bluish and
yellowish interference color which is unusual. Porphyroblastic and poikiloblastic textures. Some
are twinned. Partly replaced by chlorite and muscovite.
Replacement of biotite and staurolite by chlorite occurred as a late, retrogressive metamorphic
event. Notice that the schistosity is folded. If, as is usual, the schistosity formed parallel to the
axial planes of tight folds, there must have been a second, later period of folding. Is the staurolite
pre- or post-kinematic?
Name _______________________
Petrology — Spring 2006
Kyanite zone
#84. Quartz-kyanite rock (too aluminous to be called a quartzite; lacks foliation necessary to
call it a schist).
Quartz — <35
Muscovite — 15
Plagioclase ?: considerable untwinned plagioclase is present
Kyanite — 45: high relief, well-developed cleavage; biaxial (-); high 2V; elongated crystals with
inclined extinction. The extinction appears parallel in orientations that display the cleavage
most clearly. Very weak blue color. Moderate birefringence.
Ilmenite — 3
Rutile — trace: associated with ilmenite; yellow-brown color
Calcite — 1: secondary
Tourmaline: variable abundance from slide to slide
The rock is unfoliated. The calcite probably is secondary (i.e., introduced long after the rock last
equilibrated), for it would not have coexisted with an aluminum silicate mineral in a high-grade
metamorphic rock. Instead, calc-silicate minerals would have formed.
Name _______________________
Petrology — Spring 2006
Sillimanite zone
#86. Sillimanite-K feldspar gneiss, of Paleozoic age, Belfast area, Maine. A very weakly
foliated rock.
Quartz — 12
Plagioclase? — 20: untwinned
Cordierite — 5: distinguished from the plagioclase only where there are pleochroic yellow halos
around very minute inclusions. Thus its modal abundance is uncertain.
Microcline — 16
Muscovite — 10
Biotite — 15
Sillimanite — 20: both fibrolitic and coarse sillimanite are present
Pyrrhotite — 2
Slide e contains minor hercynite ( Fe-Al spinel ) in one crystal of biotite. Do not bother looking
for it if you did not happen to get slide #e. Slide e also contains a possible trace of garnet in the
muscovite. Slide d contains minor rutile.
Sillimanite and K-feldspar are found together only in very high grade metamorphic rocks, in
which the following (simplified) reaction has occurred:
muscovite + quartz
K-feldspar + sillimanite + H2O
The fibrolitic sillimanite likely was produced by the above reaction; the coarse sillimanite in this
rock likely was produced by inversion of andalusite. Further increase of temperature leads to the
complete breakdown of micas and other hydrous minerals, under the conditions of the pyroxene
granulite facies. Notice that the rock is essentially unfoliated, but there is a slight orientation of
the sillimanite; for that reason, the rock is called a "gneiss", but it is not a very respectable
example of a gneiss.